With the widespread proliferation of cellular phones and notebook personal computers in recent years, there is a strong desire for small-sized secondary batteries with high capacity. Under such background, nickel-metal hydride storage batteries with high reliability and high capacity have been rapidly spread. Meanwhile, a nickel-cadmium battery which includes an aqueous electrolyte and is inexpensive is commonly employed as the secondary battery used for power tools such as electrical drills and electrical screwdrivers because the importance is put on applicability to super quick charge and large current discharge. Recently, however, taking environmental concerns into account, the use of nickel-metal hydride storage battery without cadmium has been adopted instead of the nickel-cadmium battery with negative electrode containing cadmium. Nickel-metal hydride storage battery comprises a positive electrode plate comprising a nickel porous substrate or the like retaining a positive electrode active material mainly composed of nickel hydroxide, a negative electrode plate comprising a punched metal or the like retaining a negative electrode material mainly composed of a hydrogen storage alloy, a separator interposed between the positive and negative electrode plates and an alkaline electrolyte.
In the nickel-cadmium battery, a sintered type positive electrode plate is widely used because it is suitable for large current discharge and has good durability. The sintered type positive electrode plate is produced by impregnating a sintered nickel substrate with nickel hydroxide. The sintered substrate within the positive electrode plate also contains a small amount of cadmium. The cadmium is substituted for nickel ions within the crystal of the nickel hydroxide, or exists outside of the crystal of the nickel hydroxide as cadmium hydroxide.
In the cadmium ions substituted for the nickel ions within the crystal, the bivalent state is most stable and the valence does not vary even during discharge. The cadmium ions have an effect to suppress the swell and degradation of the positive electrode plate because the cadmium ions suppress increase in the valence of the nickel ions to about tetravalent in charge reaction and also suppress the reaction which produces γ-nickel oxyhydroxide (effect 1).
The cadmium hydroxide which exists outside of the crystal of the nickel hydroxide has an effect to increase the overvoltage of the oxygen generation reaction during the charge of the positive electrode plate to improve the charge efficiency (effect 2). It is considered that this involves the dissolution and redeposition of the cadmium hydroxide, but its detailed mechanism is not known yet.
The cadmium hydroxide also functions as antipolar material. To be more specific, when the positive electrode plate is overdischarged, cadmium hydroxide causes a reductive reaction:Cd(OH)2+2e−→Cd+2OH−ΔE=−0.80 V (vs SCE)thereby, the effect to suppress hydrogen generation reaction on the positive electrode plate:2H2O+2e−→H2+2OH−ΔE=−0.82 V (vs SCE)can also be obtained (effect 3).
As described above, the cadmium contained within the positive electrode of the nickel-cadmium battery along with nickel exhibits a superior effect. When the nickel-metal hydride storage battery is used instead of the nickel-cadmium battery from environmental concerns, however, it is necessary to avoid the use of cadmium in the positive electrode plate.
Accordingly, in the nickel-metal hydride storage battery, the ions of zinc or Group II elements such as magnesium having almost the same size as a nickel ion are contained in the crystal of nickel hydroxide included in the positive electrode plate in stead of the use of cadmium. The effect 3 as the antipolar material cannot be expected from these metals, but the effect 1 to suppress the swell and degradation of the positive electrode plate can be realized.
The nickel hydroxide containing metal ions such as zinc ion and magnesium ion can be filled into a substrate by immersing a sintered substrate into an aqueous solution of nickel nitrate containing zinc ions or magnesium ions and subsequently into a strongly alkaline solution. In this case, however, there arise problems that zinc hydroxide deposited outside of the crystal of nickel hydroxide is dissolved into the strongly alkaline solution or, although the details are not known, that the filling factor of the positive electrode active material is lowered because the bulk density of the active material obtained by containing magnesium ions is reduced.
Therefore, the following method is adopted to obtain the non-sintered positive electrode plate: spherical solid solution nickel hydroxide particles with high bulk density in which the ions of Group II metals are contained are prepared beforehand by reactive crystallization process in which pH, temperature and the like in the reaction vessel are controlled, and the resultant particles are filled into a foamed nickel substrate. According to this method, an effect to suppress the swell and the degradation of the positive electrode plate can be obtained and, at the same time, a positive electrode plate with higher capacity than conventional nickel-cadmium batteries can be obtained.
In order for the nickel-metal hydride storage battery to obtain the effect 2 to increase the overvoltage of the oxygen generation reaction, a trace amount of oxide of rare earth element such as yttrium oxide (Y2O3), ytterbium oxide (Yb2O3), lutetium oxide (Lu2O3) or erbium oxide (Er2O3) is added to the solid solution nickel hydroxide particles instead of cadmium hydroxide. Particularly, inexpensive yttrium oxide, which is abundant in reserve, is mostly used.
In the secondary batteries for power tools which require excellent large current discharge characteristic, various improvements in the current collecting system are necessary. For instance, there are proposed a tabless type positive electrode plate having an exposed portion of a core member (the portion where active material is not filled) at the upper or lower part thereof, the improvements in the shape of the current collector and the welding portion, and the like. It is also necessary to adopt a relatively rigid battery case and a gas exhaustion valve (valve of safety vent) with a great working pressure in order to cope with the increase in the internal pressure of the battery along with the super rapid charge.
The voltage of the nickel-metal hydride storage battery per cell is about 1.2 V. Accordingly, in order to provide the necessary voltage, an assembly of 10 to 12 batteries connected in series is usually employed for power tools. Further, in order to avoid increasing the cost and the weight and the volume of the power circuit, the protection circuit to prevent overdischarge is mostly not set in power tools. Consequently, it is surmised that the battery keeps discharge until the motor of the tool fails, specifically, until the voltage of the assembly is lowered to about 1 to 2 V. In such case, each of the cells go into an overdischarged state where the voltage is as low as 0.1 to 0.2 V even if each of the cells constituting the assembly has a uniform capacity.
At this stage, usually, a cobalt component such as metallic cobalt, cobalt hydroxide or cobalt monoxide is also added to the positive electrode plate containing nickel hydroxide as a conductive agent. A part of the β-cobalt component is electrochemically oxidized into cobalt oxyhydroxide in the initial charge after the production of the battery to form a conductive network. When the battery is kept discharged until its voltage becomes as low as the above figures, the potential of the positive electrode plate becomes lower than the trivalent/bivalent equilibrium potential of the β-cobalt oxyhydroxide (about 0.9 to 1.0 V in terms of battery voltage), and the β-cobalt oxyhydroxide is reduced to HCoO2− (cobalt complex ion), which is soluble in alkaline solution. This results in a local damage to the conductive network and a gradual reduction in the battery capacity.
In the nickel-metal hydride storage battery, a reducing atmosphere is maintained by gas phase hydrogen which is in an equilibrium state with hydrogen absorbed in a hydrogen storage alloy. Such reducing atmosphere facilitates the reduction of β-cobalt oxyhydroxide mentioned above and accelerates the damage to the conductive network.
Additionally, an initial variation and a variation with time which occur during the production process in the battery capacity, or a temperature variation in the assembly due to uneven temperature atmosphere is inevitable. Accordingly, repeated discharge of the assembly consisting of a plurality of batteries connected in series causes deterioration of the batteries due to the reverse charge. Since the nickel-metal hydride storage battery is designed to be controlled by the capacity of the positive electrode in the case of using a negative electrode comprising a common MmNi5 type alloy, hydrogen gas occurs by the polarity inversion of the positive electrode plate in the reversely charged battery. Particularly, when the battery is discharged with large current, hydrogen absorption by the negative electrode alloy is so slow that the internal pressure of the battery increases rapidly, and the gas exhaustion valve is activated. Subsequently, the electrolyte and the hydrogen gas are released to decrease the battery capacity significantly.
As mentioned above, when conventional nickel-metal hydride storage batteries are applied to power tools or the like, there is a problem that the cycle life of the battery, especially as an assembly, is poor.